The invention relates to a thread-cutting chuck for CNC (Computer Numerical Control) machine technology of rigid tapping with a clamping means held in a chuck shaft, particularly a collet chuck, for the tap.
In the case of rigid tapping, it is a question of NC machines, for which the machine manufacturer promises 100% accurate synchronization of the rotational movement of the machine spindle with the linear advancing motion as a function of the thread pitch.
Irrespective of the fact that, in the case of this process, the speed of the computer frequently limits the possible thread-cutting cycle time, experience has shown that, in the case of so-called rigid tapping, after reversal of the direction of rotation, that is, when screwing out the tap, high axial forces act on the rigidly clamped tap employed in the pulling or pressure direction without any recognizable regularity. These forces bring about an increased tool wear and decrease the surface quality of the thread that is cut.
These interfering forces addressed can be explained as follows. During the thread-cutting process, the computer of the machine control system synchronizes a linear movement of the machine with a rotational movement of the machine. Over electrical pulse generators, which are connected with the movement mechanism, the computer receives positional information regarding the respective movements. Depending on the quality of the resolution of the pulse generator, an unavoidable positional error (similar to the effect of tooth profile clearance during mechanical control) arises between the advancing unit and the workpiece spindle during the reversing of the tap. The tap, still screwed into the workpiece, becomes a tension bar or a compression bar, the forces arising increasing steeply because of the modulus of elasticity of the material of the tap.